The present invention relates to a passive electro-optic display cell comprising two glass plates spaced apart by a frame positioned between them, thus providing an enclosed space in which a liquid crystal is imprisoned. On their inner faces these plates carry control electrodes and, in each case, a layer for alignment of the molecules of the active constituent of the cell. The invention also includes a method of manufacture of such a cell.
The object of the invention is to overcome problems which the mass-production of liquid crystal display cells presents. Two types of problems are posed, firstly those which are concerned with a reduction of the necessary handling operations, in order to increase productivity, and secondly those which relate to the quality of the product obtained, especially to effective product life.
The first type of problem has already been dealt with by implementing methods enabling simultaneous production of a series of cells (see Swiss Pat. No. 11.387/78). It is to the second type of problem that the present invention is principally directed.
It is advisable to point out that the cell which forms the subject of the present invention, as well as its method of manufacture, does not rule out mass-production, quite the contrary being the case. Thus, the different layers of which the cell are composed are all applied over large surfaces, which allows them to be applied to glass plates the dimensions of which are larger than those of the plates of each cell and which are then broken up with the object of separating the individual cells which are so produced.
Frequently observed faults in passive electro-optic cells and which the invention aims to remove, or at least reduce, are as follows;
Formation of micro-areas in the display zones (segments ) producing the so-called "reverse tilt" phenomenon, which affects the uniformity of the activated segments.
Increase, with time, of the current which serves to control the display.
Loss of alignment after a long period of operation or, more rapidly, after operation at high temperatures.
The three drawbacks mentioned above are all due to the progressive dissolution of the alignment layers, especially when such layers are organic, because of the solvent effect of the liquid crystal, and due to the penetration of impurities into the liquid crystal which is inadequately protected. These impurities originate from the sealing frame of the cell, from its glass plates or from the layers which the latter carry, and they migrate by diffusion.
Another drawback which the display cells with a plastics sealing frame frequently possess is the sharp increase in the operating current which occurs when they are in a humid atmosphere, owing to the weak bonding between the sealing frame and the cell plates, which results in water molecules readily diffusing into the cell.
A loss of potential through the insulating layers which normally cover the electrode networks, to provide protection against direct current, is also detected. This loss of potential adversely affects the electrio-optic behaviour of the cell.
Finally, the formation of a marginal halo around the activated display segments, resulting in too small an angle of tilt of the molecules in relation to the alignment layer, is a further drawback which should be mentioned.